In general, inkjet printing machines or printers include at least one printhead that ejects drops of liquid ink onto recording media or onto an image receiving member surface. A phase change inkjet printer employs phase change inks that are in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. A printhead ejects drops of the melted ink to form an ink image. The ink can be ejected directly onto print media or onto an image receiving member surface, such as a rotating drum or moving belt, before the image is transferred to print media. Once the ejected ink is transferred to the print medium the ink droplets quickly solidify to form an ink image.
The media used in inkjet printers are typically provided in sheet or web form. A media sheet printer typically includes a supply drawer that houses a stack of media sheets. A feeder removes a sheet of media from the supply and directs the sheet along a feed path past a printhead so the printhead ejects ink directly onto the sheet. In a web printer, a continuous supply of media, typically provided in a media roll, is entrained onto rollers that are driven by motors. The motors and rollers pull the web from the supply roll through the printer to a take-up roll. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web. Along the feed path, tension bars or other rollers remove slack from the web so the web remains taut without breaking.
The processing of a print medium often continues after an ink image is printed onto a media sheet or media web. For example, in a cut sheet printing system, the media sheets can be sorted, collated, stapled, bound, or otherwise organized after ink images are printed on the media sheets. For a continuous media web, additional processing includes a cutting process whereby sections of the media web are cut into individual pages such as pages in a book or magazine. Further, some printers generate multiple printed pages across a width of a print medium that correspond to two or more tandem pages. For example, in a “two up” print mode, two pages are printed in tandem across a width of a single media sheet or a media web. The print medium is cut longitudinally to separate the tandem pages. Various devices that are referred to as “finishing units” perform the additional processing on the print medium after the inkjets have formed the ink images on the print medium.
One challenge that arises with the use of finishing units is that some ink in the ink images may offset from the print medium and transfer onto a roller, baffle, or other component in the finishing unit. Ink offset reduces the quality of printed images in at least two ways. First, an ink image that experiences offset has a degraded image quality because the print medium loses a portion of the ink in the ink image. Second, the offset ink may re-transfer onto another page and contaminate the ink image on the other page. Consequently, improvements to inkjet printers that reduce or eliminate the offset of ink from printed ink images would be beneficial.